Semester

Spring

Date of Graduation

2011

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Lane Department of Computer Science and Electrical Engineering

Committee Chair

Xian-An Cao

Abstract

White light sources based on III-Nitride light-emitting diodes (LEDs) hold great promise for developing energy-efficient solid-state lighting (SSL) technologies. However, the optical output power of InGaN-based green and blue LEDs at high driving currents is limited by quantum efficiency (QE) droop and low light outcoupling efficiencies. The former is a phenomenon that LEDs suffer a decline in quantum efficiency as the driving current increases, and prominently occurs in LEDs with high In contents. Poor light outcoupling is a challenge facing LEDs of all colors. Due to a large contrast of refraction index between nitrides and air, the majority of photons generated in the active region are trapped inside LED chips and reabsorbed. To develop powerful LED lighting sources capable of high-current operation, new strategies must be developed to overcome the droop obstacle and improve the efficiency of light extraction.;The objective of the current work is to improve the external quantum efficiencies of InGaN-based green and blue LEDs from these two aspects. The first part of this dissertation presents studies of the underlying mechanisms of efficiency droop in InGaN-based LEDs and our effort to develop LEDs with reduced droop through structure optimization. We investigated the optical characteristics of InGaN-based multiple-quantum-well (MQW) LEDs on sapphire with peak emission ranging from green to ultraviolet over a wide injection range. The current dependence of both the QE and peak shift appears to be a strong function of the In content in the active region. The QE of the green LED peaks at a current density as low as 1.4A/cm2, and decreases dramatically as current is increased, whereas the In-free deep-UV LEDs have a nearly constant QE at currents up 1 kA/cm2. To understand the role of threading dislocations in the droop behaviors, green LEDs were grown and fabricated on free-standing GaN. The density of microstructural defects in the LED structure was substantially reduced, leading to a significant reduction in defect-assisted tunneling currents a ∼65% peak internal QE. However, it suffered from even more dramatic efficiency droop which occurs at a current density as low as 0.3 A/cm2. These results offer a strong support for the argument that carrier overflow from localized states and loss at interfacial misfit defects is the nonthermal mechanism of the efficiency droop. Reduction of misfit defects in green LEDs by using an strain-compensated InGaN/InGaN MQW structure led to reduced efficiency droop at high currents, suggesting that strain engineering provides a feasible solution to the droop problem.;The second part of the dissertation explores the applicability of photonic crystals (PhCs) as diffraction gratings for light outcoupling from InGaN-based LEDs. Time-domain modeling of GaN PhCs with triangular lattices was carried out to determine the photonic band structure and the desirable geometry of 2D PhC slabs for light-coupling. 2D PhC slabs of a triangular lattice with the lattice constant of 333 nm and airhole diameter of 200 nm were fabricated on top of blue LEDs using e-beam lithography and inductively-coupled plasma etching. A 50% enhancement of electroluminescence was achieved compared to similar LEDs without integrated PhCs. In order to minimize the adverse effects of sidewall plasma damage which lead to increased junction leakage and nonradiative recombination rates. We developed a new post-etching treatment technique which combined thermal annealing and sulfide passivation. Blue PhC LEDs subjected to annealing at 700 °C followed by (NH4)2S passivation exhibited a complete restoration of the electrical characteristics.

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